The invention relates to a device for contact-free thickness or interval measurement comprising a light source for the generation of a sharply concentrated light ray beam in a given direction. A beam deflector is provided for the periodic movement of the light ray beam at right angles to the given direction. A beam divider is connected to the beam deflector, and first and second light-sensitive detectors are positioned at a region where the light ray beam is divided out of the given direction by means of the beam divider. Each detector only registers light from a specific direction (detector with an optical axis). At least one third light-sensitive detector is provided whose optical axis intersects the light ray beam at a zero plane. An electronic evaluation unit generates from the detector signals a control voltage corresponding to the excursion or deflection time of the light ray beam between the first and second detector and at least one voltage pulse which is a measure of the distance of a surface of a measured object from the zero plane. An impulse/digital converter is also provided to which the voltage pulse is supplied, and a digital/analog converter is connected thereto whose reference voltage can be connected through in accordance with the output value of the impulse/digital converter.
In a known device of this type (Siemens Forschungsund Entwicklungsbericht, Vol. 6 (1977), No. 3, pages 180-188), incorporated herein by reference, the thickness or interval measurement is based on the determination of the deflection or excursion time which the light ray beam requires in order to arrive from a specific point on the zero plane to a specific point on the measured object. The deflection or excursion time does not depend solely on the interval between the two points, but, rather, also depends on the deflection velocity of the light ray beam. Since the deflection of the light ray beam is periodically repeated, the deflection velocity depends both on the deflection frequency as well as on the deflection amplitude. This means that each change of the deflection frequency or of the deflection amplitude changes the deflection velocity, and, thus produces a measuring fault. Given rapid measurements in which the measured value must be determined at each individual excursion, this measuring fault can also not be entirely avoided by means of a stabilization circuit for the deflection velocity.
It is already known from the report discussed above to eliminate the measuring fault caused by the change of the deflection velocity by connecting the measured value during evaluation according to the following equation: EQU .DELTA.t.sub.dk =(.DELTA.t.sub.N /.DELTA.t.sub.BA).DELTA.t.sub.d ( 1)
Here, .DELTA.t.sub.N is a time standard; .DELTA.t.sub.BA is the time which the light ray requires in order to arrive from a first photo diode which determines the zero plane to a second photo diode which specifies the excursion or deflection time; .DELTA.t.sub.d is the actual and .DELTA.t.sub.dk is the corrected measuring time. This is calculated after digitalization of the measured times with the assistance of the microcomputer.
For most applications, it is necessary that the measured times exist not only in digital, but also in analog form, so that a direct control, for example, of the amplitude of the light ray beam or--upon employment of such a device for monitoring the thickness of rolled goods--for the control of the drum interval can be undertaken.
In many cases, not even the precise calculation of the measured thickness or of the measured interval is required. It is precisely in such cases that a microcomputer for the correction of the measuring time would represent a significant additional expense.